Distributed Computing Architecture based on Geo Services; A Loosely Coupled Method for Linking GIS and Environmental ModelsTo guarantee interoperability, both provider and requester must know what to send and what to expect. SOAP is a lightweight, message-based protocol to exchange information between nodes in a decentralized, networked environment built on XML and standard Internet protocols, such as HTTP. The SOAP protocol specification defines an XML structure for messages (the SOAP envelope), data type definitions, and a set of conventions that implement remote procedure calls and the format of any returned data (the SOAP body). As shown in Fig. 4, a SOAP message consists of header and body information.  Fig.4: SOAP Message 4.5. Discovery (UDDI (Universal Description, Discovery, and Integration)) Catalog service is a component in service oriented architecture to discover the types of service and data and their relevant instances. A catalog service plays a ‘directory’ role in helping providers to describe and advertise the resources availability and requestors to discover the right resources. UDDI provides three basic functions, popularly known as publish, find, and bind:
 Fig.5: The basic model of service and the elements of Web services stack. http://en.wikipedia.org/wiki/Web_service 5. Geo-Services According to definition from ISO 19119 geo-services can be defined as a collection of geo-operations, accessible through an interface [ISO, 2001]. For example The Environmental Easements web service contain geo operations such as GetCentroids returns the polygon centroid, LatLonExtentForOrtho returns the spatial extent for the orthophoto image in terms of latitude and longitude, GetEasementFromTerraServerArea returns polygon data for the easements within the extent of the TerraServer ortho image area and so on [Barclay et al, 2002]. The geo-services can be categorized into geo data services which are tightly coupled with specific data sets and offer access to customized portions of that data and geo operation services which provide operations for processing or transforming data in a manner determined by user-specified parameters. One of the tasks which are needed by environmental modeler is to visualize geo data or physical fields which are represented by different maps or raster from multiple dissimilar servers. The OpenGIS Web Map Service Interface Implementation Specification offers a way to enable the visual overlay of complex and distributed geographic information maps simultaneously, over the Internet. In contrast the geo operation services are not associated with specific data sets. [Alameh N., 2003]. A geo operation is defined by its inputs, outputs and function. The operation uses a certain geo operation algorithm to derive new data from data inputs. The outputs usually represent a new geospatial data product. 6. Implementation of a WMS The OpenGIS Web Map Service Interface Implementation Specification offers a way to enable the visual overlay of complex and distributed geographic information maps simultaneously, over the Internet. In the context of WMS a "map" is a raster graphic "picture" of the data rather than the actual data itself [Cookbook, 2003]. The Web Mapping Server (WMS), which produces maps as two-dimensional visual portrayals of geospatial data performs three operations including produce a map, answer queries and tell other programs about maps it can produce by providing three types of interface which are called GetCapabilities (required), GetMap (required), and GetFeatureInfo (optional) [OGC, 2000].  Fig.6: Architectural view and main Component of WMS The diagram illustrated in Fig 6 presents the architectural view and main components of web map services. In this case, environmental modeler as a client sends a request through his/her web browser to a server which is called web server. The web server understands requests and how to fulfill them. The web server sends the respond to the web browser according to which of operations mentioned in the request. The sending request and response can be done through a communication protocol called HTTP (over TCP/IP). If a request is sent to a Web server using vocabulary it doesn't understand, it will respond with an error. In essence, the WMS Specification defines the vocabulary and the syntax of the commands/operations that enable Web servers and clients to communicate over the HTTP protocol. 6.1. Web Map Server Implementation According to architecture view the main components of Web Map Server are Web server, Web map services and field data or raster map. A web server is a server that understand request and send response through the HTTP. We installed the Apache HTTP server (http://www.apache.org/) which implements version 1.1 of the protocol, referred to as HTTP/1.1. On Windows, Apache is normally run as a service on Windows NT, 2000 and XP. For implementing WMS it is needed to write a CGI (Common Gateway Interface) program. It defines a way for a web server to interact with external content-generating programs, which are often referred to as CGI programs or CGI scripts. An example of CGI program which has been developed in order to bring a chunk of field data or raster map to the web browser can be found in http://www.intl-interfaces.com/cookbook/WMS/basic-wms2.py. A CGI program must be preceded by a MIME-type header. MIME-type is a way to describe the kind of document being transmitted. Its name comes from that fact that its format is borrowed from the Multipurpose Internet Mail Extensions. This is HTTP header that tells the client what sort of content it is receiving. The output needs to be in HTML, or some other format such as gif image, or other non-HTML content that a browser will be able to display. In our case study we used the image of the world as sample of field data or raster map. 6.2. Client Side In the client side, there is no need to install additional program. An environmental modeler can send a request through his/her web browser. The response which is sent by server is represented in web browser. For example the following hypothetical URL requests the capabilities of web map services in a server called localhost: http://localhost/test/basic-wms2.py?request=capabilities&wmtver=1.0.0 The result of GetCapabilities operation is illustrated in Fig 7:  Fig.7: The result of GetCapabilities request on web browser And the following hypothetical URL requests a chunk of image on that server: http://localhost/test/basic-wms2.py?WMTVER=1.0.0&REQUEST=map&LAYERS=RELIEF&STYLES=default&SRS=EPSG:4326&BBOX=-45.0,24.0,63.0,40.0&WIDTH=256&HEIGHT=128&FORMAT=JPEG The result is depicted in the following figure:  Fig.8: The result of GetMap request 7. Conclusion In this paper we discussed that environmental modeler needs to use GIS capabilities including geo data and geo services in order to visualize and process physical fields used in model. In this regard we presented different linking methods. As consequence, the linking between GIS and environmental model based on web services which use loose coupling interaction satisfies the communication between modeler and GIS. The OpenGIS WMS as geo data services contains operations for visualizing physical fields. In these types of services the data is tightly coupled to service. However in addition to visualization of physical fields, modeler needs geo operation services in order to get new physical field value from old data. Semantic ambiguities are barriers against efficient use of these types of services. The future work is to study these semantic ambiguities and propose a solution for formal description of geo operation services. References
|